Rotary wiped-film evaporator



Nov. 26, 1968 J. DONOVA'N ROTARY WIPED-FILM EVAPORATOR Filed April 24, 1967 I w m N w mm m. 1 m I vD w w m s E \H\\\\ w fl. a M \m m 3, m m E1 ATTORNEY Qmmm United States Patent 3,412,776 ROTARY WIPED-FILM EVAPORATOR James Donovan, Cambridge, Mass., assignor to Artisan Industries, Inc., Waitham, Mass., a corporation of Massachusetts Filed Apr. 24, 1967, Ser. No. 633,203 8 Claims. (Cl. 159-6) ABSTRACT OF THE DISCLOSURE A tapered horizontally-axised thin-film evaporator having projections at the peripheral edges of the rotor blades just before or adjacent the product outlet which projections generate a hydraulic weir. The weir creates a thicker film just before the product outlet, which film enables the feed material being processed to be carried by the blades to the top of the evaporator and there replenish the material being evaporated. The feed material without force is introduced at the feed end, at or slightly below the rotor shaft to form a liquid head which overcomes the frictional loss across the evaporator to the product outlet caused by the tapered floor.

Background of the invention In the field of wiped thin-film evaporators one design currently in use is a tapered thin-film evaporator with the internal circumference of the processing chamber gradually decreasing as it approaches the product end of the evaporator. In processing material in this type of evaporator, a retardation effect on the process material is encountered in that the material has to flow up the tapered fioor of the evaporator. This sloping floor also causes a gradation or a tapering off of the thin film created between the peripheral edges of the rotor blades and the interior wall of the processing chamber. The thin film created at the bottom of the evaporator at the feed end will be equal in thickness at least to the distance between the edge of the rotor blade and the interior wall of the chamber. However, as the thin film progresses towards the product outlet, the film then becomes progressively thinner or diminishes as it approaches the product outlet. This effect generally causes the tapered unit to run dry, where the liquid film at the top portion of the evaporator tends to dry because of the very thin film at the bottom portion of this end of the evaporator, that is there is not enough of a pool of liquid material to be caught, or taken up by the blades to replenish the material which is being evaporated at the top of the evaporator. One means suggested to overcome this problem is disclosed in US. Patent No. 2,927,634 issued Mar. 6, 1960. In this method, the feed material is forced into the top of the tapered evaporator to overcome the retardation effect. Another means suggested is to tip the evaporator from the horizontal to help the thin film flow to the discharge end or the product outlet of the evaporator. A further means suggested is to adjust the clearance between the peripheral edges of the rotor blades and the interior wall of the processing chamber by axial movement of the rotor shaft. Usually, when this clearance is cut down the amount of hold up time, in the evaporator increases.

Summary of invention I have discovered a novel method and apparatus for overcoming the problems associated in the past with tapered horizontally-axised thin-film evaporating units. In my apparatus the material to be processed is introduced directly into the feed end of the tapered thin-film evaporator at a point upstream of the processing section as defined by the rotor blades, sufiicient to form a liquid head of feed material higher than the product outlet and to overcome the frictional losses between the feed and product outlets. For example, the feed material is introduced directly, either into the rotor shaft or at a point slightly below the rotor shaft. In this manner, a liquid head is created at the feed end of the evaporator which is sufficient to overcome height difference and frictional loss between the feed end and product outlet and therefore the material does not have to be forced by pump or vacuum into the evaporator. Another novel feature of my invention is to place raised surface projections, such as wedges, at the peripheral edges of the rotor blades adjacent to the product outlet. These projections will extend generally about one half the distance between the peripheral edge of the rotor blade and the interior wall of the processing chamber e.g. inch. The projection then acts as a pump before the product outlet to create a hydraulic Weir. The projection in this manner increasesthe material holdup just before the upstream face of the projection and provides a thick layer of material adjacent the product outlet and a thinner non-dry layer before the product outlet. This results in a generally more uniform thin film along the entire downstream length of the tapered thin-film evaporator. Also, this prevents or inhibits the tendency of the tapered unit to run dry at the top of the discharge end, since the thin film at the product end is thicker and the blades have more material through which to cut and therefore replenish the liquid being evaporated at the top of the discharge end.

Accordingly, it is an object of my invention to provide an improved tapered thin-film processing apparatus wherein the material to be processed need not be forced into the evaporator but is introduced in such a manner as to create a liquid head which insures that the material will flow through the evaporator, overcoming the frictional loss across the evaporator.

Another object of my invention is to provide an improved tapered thin-film evaporator wherein projections or wedges are secured to the peripheral edges of the rotor blades adjacent the product outlet to generate a hydraulic weir thereby creating a uniform thin-film thickness along the length of the processing section of the evaporator. These and other objects of my invention will be apparent to those skilled in the art from the following description and the accompanying diagram wherein:

Brief description of the drawing FIG. 1 is a horizontal cross-sectional view of the tapered thin-film evaporator;

FIG. 2 is a fragmentary sectional view of the wedges used in my invention;

FIG. 3 is a fragmentary top sectional view of the wedges;

FIG. 4 is a further embodiment of my invention employing a raised interior product outlet.

Description 0) the preferred emb0diment(s) My invention comprises in combination a tapered horizontally axised thin-film evaporator 10 with a conical processing chamber 12 having a feed inlet 16 for the introduction of the material to be processed and a product outlet 18 for the removal of the product material. The processing chamber is surrounded to all or a greater part of its length with a temperature control jacket 14 for the introduction of steam or a similar heat exchange material. Adjacent the processing chamber 12, is a vapor chamber 22 with a vapor outlet 20 for the removal of vapors. Passing through evaporator 10 is a horizontal rotor shaft 24 secured at either end by closing heads 28 and 26, bearings 34 and 36 and suitable packing materials 30 and 32. Secured to the rotor shaft 24 and coaxially and radially extending therefrom are a plurality of rotor blades 28, the edges of which upon rotation of the rotor shaft 24 create a thin film of the material to be processed against the interior wall of the processing section 12. Secured to the ends of the rotor blades adjacent the product outlet are wedges 30 which extend about onehalf the way between the interior wall of the processing chamber 12 and the peripheral edges of the rotor blades 28.

In the operation of my evaporator, the material to be processed such as an organic solvent containing an ester plasticizer or polymer to be concentrated is introduced by gravity into the feed inlet 16. The feed inlet is located just below the rotor shaft 24 but of sufiicient height to create a liquid head 32. Introduction of the solvent by gravity and coaxially with rotor shaft 24 avoids the need for a pump to overcome the rotational force in the blade processing section. Upon rotation, the rotor blades create a wiped turbulent thin film of the material being processed against the interior wall of the evaporator. Steam is introduced into the temperature control jacket 14 thereby evaporating the material on the interior wall of the chamber 12. The vapors flow into the vapor chamber 22 and out the vapor outlet 20. As the material is processed, it is concentrated and flows toward the discharge end of the evaporator and out the product outlet 18. The liquid head 32 is created at the feed end of the evaporator which is 25 sufficient to enable the thin film being processed to overcome the frictional loss between the feed end and the product outlet 18.

In addition the wedges secured to the rotor blades 28 generate a hydraulic weir which the thin film must 3O creates a liquid layer of sufiicient thickness which enables the blades to carry liquid to the top section of the evaporator at the discharge end to replenish the liquid that is being evaporated at the discharge end.

The wedge 30 is shown more clearly in FIGS. 2 and 3 where the wedge has a tapered surface which surface is,

sloped so that the liquid film being processed, upon contacting the sloped surface, will be given a vector component in a axially rearward direction or back toward the feed end of the evaporator. Of course, it is understood that a wedge or projection of any shape and size, as long as it generates a hydraulic weir, would be suitable for the purposes of my invention. Another advantage of my wedge 30 is that it forms just before the product outlet, a restriction which the material must pass through in a thinner film thereby improving processing characteristics.

For the purposes of this application the word projection is intended to include any structure either on the rotor blade or between the rotor blades which structure extends into the space between the peripheral edges of the rotor blades and the interior wall of the chamber. For example an arcuate segment could extend between two blades or between all blades forming a continuous band or ring the outer edge of which extends into the space between the peripheral edges of the blades and the interior wall of the chamber. Also, the inner surface of the chamber may be machined at one end providing an annular dam to allow close tolerance between the projection(s) and the inner wall of the chamber, or a band may be inserted in the one end of the chamber to provide a close tolerance between the projection(s) and the inner surface of the band.

In my invention the feed is introduced into the evaporator axially to the rotor shaft and between the interior wall of the closing head at the feed end, and the adja- 70 cent ends of the rotor blades, this enables the feed to be introduced without force if desired. If the feed is introduced at a point when it initially must overcome the centrifugal force created by the rotor blades say at the top portion of the evaporator in the blade processing section,

then a pump or other means must be used to force the material into the processing section.

Another significant advantage in my method of introducing the feed is the forming of the liquid head 32 at the feed end of the evaporator which head is sufficient to move the material through the evaporator, and an additional pumping or other force is not necessary. The feed material should be introduced in the evaporator at a point high enough to create a sufficient liquid head to overcome the frictional loss between the product outlet and the feed end. As shown and described the feed inlet 16 is slightly below the rotor shaft. It should be understood that the feed may be introduced into the rotor shaft for example, if a tubular shaft with flow passages or openings therein is used, or it may be introduced above the axis of the rotor shaft, as long as it is of sufiicient height so that it does not have to be forced into the evaporator to create the desired liquid head. Therefore in my invention the feed does not have to be forced into the evaporator to overcome either the centifugal force created by the rotor blades or the frictional loss across the processing section.

A further embodiment of my invention is shown in FIG. 4 where the product outlet 18 extends slightly, a predetermined amount e.g. /2", into the body of the evaporator so as to form a weir. This raised product outlet creates a head of liquid at the downstream side of the product outlet, and thus tends to increase the feed material pool through which the blades cut whereby the drying out in the top portion of the evaporator is prevented or inhibited.

The three features of my apparatus are all directed toward a common problem, improving the processing characteristics of thin-film evaporators. Accordingly the introduction of the feed material without force, the use of wedges and the raised product outlet may be used collectively, individually or in any combination thereof. It should be noted that the location and shape of the vapor chamber is not critical.

Although my invention has been briefly described with reference to a tapered evaporator it is obvious that the concept is applicable to any 'horizontally-axised thin-film evaporator wherein the material being processed must flow up an angular or inclined path to reach the product outlet. For example, a horizon-tally-axised thin-film evaporator having a cylindrical or straight sided chamber which is inclined from the horizontal and in those tapered units where the rotor shaft is axially adjustable may be used.

In designing and fabricating thin-film evaporators the inner surface of the chamber is sometimes machined. This allows for a close tolerance between the peripheral edges of the blades and inner surface of the chamber. If the chamber is formed from an unmachined surface the tolerance between the edges of the blades and the inner surface must be greater to allow for the coarseness of the surface. However the cost of building an evaporator with an unmachined surface is less than and the heat transfer is greater than with a machined chamber, because with a machined surface a thicker plate is used initially and machined to a smooth surface and the average thickness is greater and therefore the heat transfer is less.

The technique of having a projection extended beyond the peripheral edges of the rotor blade is especially useful when a large clearance, say between about A and is used which is generally necessary when the interior wall of the processing chamber is unmachined. Therefore my invention is particularly advantageous when a large clearance between the peripheral edges of the blades and the interior wall of the chamber is used.

My invention as described and illustrated overcomes many of the difliculties encountered in the past with conventional horizontally-axised tapered or inclined axised straight sided thin-film evaporators by providing a means whereby the feed material is introduced into the evaporator without force, a uniform thin-film thickness is maintained along the entire length of the evaporator and the a tendency for the material being evaporated at the top portion of the evaporator at the product end to dry out is prevented or inhibited by replenishing the liquid as it is evaporated.

What I claim is:

1. A horizontally-axised fluid processing apparatus of the rotary thin-film type which comprises in combination:

(a) a horizontally axised chamber characterized by an interior frusto-conical wall;

(b) a rotor shaft within the chamber;

(c) means to rotate the rotor shaft;

(d) rotor blades secured to the rotor shaft for rotation therewith, the blades generally radially and coaxially arranged from the rotor and extending into a close relationship with the interior wall of the chamber forming a small space during operation between the peripheral edges of the blades and the interior wall;

(e) a feed inlet at the large end for the introduction of material to be processed;

(f) a product outlet at the other end for the removal of processed material from the chamber which outlet is higher than the floor of the chamber at the opposite end thereof;

(g) means secured to the rotor blades immediately upstream of the outlet for creating a hydraulic weir action, said means including a projection on at least one rotor blade, said projection extending slightly beyond the circumferential peripheral edge of the rotor blade toward but not contacting the interior wall of the chamber, the projection shaped to create a hydraulic weir retarding action against progressive flow of the feed material through the chamber which weir action the material must overcome and flow past in operation to be discharged from the product outlet.

2. The apparatus of claim 1 wherein the feed inlet is in the large diameter base and at a radius from the axis such that a head of the feed material is created at the large diameter end which minimizes the need for pumping force.

3. The apparatus of claim 1 wherein the rotor shaft is adapted for axial adjustment whereby the clearance between peripheral edges of the rotor blades and the interior wall of the chamber may be varied.

4. The apparatus of claim 1 wherein the raised projection is a tapered wedge which converges in the upstream direction.

5. The apparatus of claim 1 wherein the product outlet extends slightly into the evaporator chamber to create a weir of material about the outlet.

6. A horizontally-axised fluid processing apparatus of the rotary thin-film type which comprises in combination (a) a chamber characterized by an interior frusto-conical wall and having a large diameter base and a small diameter base;

(b) a concentric rotor shaft within the chamber;

(c) means to rotate the rotor shaft;

(d) rotor blades which are tapered from the one large diameter to the other small diameter end thereof, the blades generally radially and co-axially arranged from the rotor shaft and extending into a close relationship with the interior wall of the chamber to form a small space during operation between the peripheral edges of the blades and said interior wall;

(e) a product outlet at the small diameter end for the removal of product material;

(f) a feed inlet positioned at the large diameter base at a radius from the axis which is less than the radius of the blades at the outlet for introduction of the feed material into the processing chamber whereby the feed may be introduced without force and create a liquid head at the feed end of the evaporator, which head is sufficient to overcome the frictional loss across the chamber from the large diameter end to the product outlet and avoid the centrifugal force created by the rotor blades.

7. The apparatus of claim 6 wherein a projection is located at the end of the rotor blade.

8. The apparatus of claim 7 wherein the feed is introduced below the centerline of the rotor shaft axis.

References Cited UNITED STATES PATENTS 1,284,074 11/1918 Du Pont 1596 2,774,415 12/ 1956 Belcher 159-6 3,124,624 3/1964 Berlien et al 264-13 3,348,600 10/1967 Monty 1596 3,349,828 10/1967 Monty 1596 NORMAN YUDKOFF, Primary Examiner. J. SOFER, Assistant Examiner. 

